1. Field of the Invention
This invention relates to an automatic focusing device for a video camera which accurately focuses an image of an object to be photographed on a predetermined position when the video camera is focused.
2. Description of the Prior Art
Various automatic focusing devices for cameras having hitherto been proposed and put into practical use in video cameras and still cameras.
One effective focusing device applicable to a video camera, projects a distance measuring signal, such as an infrared ray or an ultrasonic wave, to an object to be photographed (hereinafter called the object), and receives the distance measuring signal reflected from the object, and drives a camera lens toward the proper focusing position on the basis of the distance measuring signal (such a system will hereinafter be called an active focusing system). The principle of the active focusing system using an infrared ray is shown in FIG. 1, in which the infrared ray for measuring distance is projected from the projector 1, reaching an object 3 through a converging lens 2, and thereafter is reflected from the object 3 so as to be incident on a light receiving element 5 through a converging lens 4. If the distance between the projector 1 and the object 3 or the distance between the light receiving element 5 and the object 3 varies, then the angle of incidence of the reflected light on the light receiving element 5 or the position of the stop of reflected light on the surface of the light receiving element 5 varies. A computing unit 6 executes a predetermined computation based on the angle of incidence or the position of the spot of reflected light. A lens drive unit 7 drives a photographic lens 8 comprising a plurality of lenses (hereinafter called merely the photographic lens) to a proper focusing spot on the basis of the computation by the computing unit 6 so as to thereby complete the focusing.
FIG. 2 is a schematic representation of a conventional example of an active system automatic focusing device using an infrared ray, in which an infrared light emitting diode 9 (hereinafter called the infrared LED) projects the distance measuring infrared ray modulated by a constant frequency or projected intermittently, by a LED driving device 10, thereby distinguishing the emitted infrared ray from an ambient noise infrared ray. The distance measuring infrared ray passes through a converging lens 11 and reflected by an object (not shown). The reflected light passes through a converging lens 12 to form a spot of light on a photodiode 13 divided in two (hereinafter called the two-divided PD). The two-divided PD 13 outputs signal currents I.sub.1 and I.sub.2 of intensities corresponding to the spot of light on the surface thereof. In other words, when the spot or light 14 shifts upwardly from the parting line 16 of the two-divided PD 13 as shown in FIG. 3-(a), the relationship of intensities between the signal currents I.sub.1 and I.sub.2 is given by the following inequality: EQU I.sub.1 &gt;I.sub.2 ( 1)
When the spot of light 15 is centered just on the parting line 16 of the two-divided PD as shown in FIG. 3-(b), the aforesaid relationship is given in the following equation: EQU I.sub.1 =I.sub.2 ( 2)
When the spot of light 17 shifts downwardly from the parting line 16 as shown in FIG. 3-(c), the relationship is given by the following inequality: EQU I.sub.1 &lt;I.sub.2 ( 3).
Referring again to FIG. 2, the photocurrents I.sub.1 and I.sub.2 output from the two-divided PD 13 are converted by first and second signal processing devices 18 and 19 into voltages V.sub.1 and V.sub.2 proportional to the intensity of photocurrents I.sub.1 and I.sub.2 respectively. A comparator 20 compares the voltages V.sub.1 and V.sub.2 and a decision device 21 controls a lens driving device 23 to move a focusing mechanism 25 for a photographic lens 22 (schematically shown by a single convex lens) forwardly or rearwardly on the basis of the output of comparator 20. A photodiode moving device 24 moves the two-divided PD 13 in the directions of the arrows in FIG. 2 in association with the focusing mechanism 25, so that the focus adjusting mechanism 25 is stopped at the position where the photocurrents output from the two-divided PD 13 become equal to each other to thereby materialize the automatic focusing. In this conventional example, the mounting position and a mechanism movement of the two-divided PD 13 are pe-adjusted so that a shift of photographic lens 22 (of the focusing lens group) and that of two-divided PD are adapted to have a ratio of 1:1 therebetween.
Accordingly, the decision device 21 controls the moving direction of the photographic lens 22 so that the photocurrents I.sub.1 and I.sub.2 outputted from the two-divided PD 13 become equal to each other and stop the lens at the proper focusing spot, thereby realizing automatic focusing.
The conventional example is an effective focusing method having a characteristic as the active system focusing device, in which the distance measuring accuracy is hardly affected by the intensity of illumination of the object or by contrast, and the use of an infrared LED provides a sharp directivity. However, it is necessary for the video camera to photograph an object in continuous motion, whereby the aforesaid example must always continuously project the infrared ray while the photographic lens is moving toward the proper focusing spot. Hence, in a case where a moving range is large (for example, it takes several seconds to change the focus from that of an object at a short distance to that of an object at a long distance), a large current consumption is accompanied by the movement. Usually, for the use of the conventional example, a large current of 100 to 300 mA is always consumed during focusing the photographic lens, which is a serious problem from the viewpoint of portability for the video camera focusing device which is often powered by batteries. Also, it is necessary for the photodiode moving device to move in association with the focusing device like a helicoid of the lens, so that the photodiode mounting position largely restricts the design of mechanism for the photographic lens in the video camera.